The idea of an international book festival in Tehran might sound incongruous. Leaders of the Islamic republic are no great book enthusiasts. Numerous writers are banned and as one of the world's most-censoring countries, Iran has a relatively low level of book reading.

But Tehran's international book fair, held annually in the first half of May, attracts half a million visitors per day. The figure is more than the number of people who visit Frankfurt Book Fair, which claims to be the biggest in the world, over its entire duration.

In a labyrinth of identical stalls, more than 2,000 publishing houses stand next to each other in alphabetical order. Crowds flock to the stalls, sometimes pushing each other to catch a better glimpse of newly released books on shelves. Titles range from the Farsi translations of works by JG Ballard, Fyodor Dostoevsky and Haruki Murakami to the new reprints of the Qur'an and books about the significance of Imam Mahdi, the Shi'i Islam's messianic figure.

For those interested in the extraordinary events that have been sweeping across the Arab world in the past year, there's a special section called "Islamic awakening", a term Iranian leaders use to describe the Arab spring, which they say is reminiscent of Iran's 1979 Islamic revolution.

The fair's venue is an unlikely setting. Due to its vast open space in the heart of the busy capital, Tehran's enormous Grand Mosque Mosallah, a typical venue for worshippers and state religious ceremonies, has been forced in recent years to open its doors to book lovers. This is to the dismay of religious fanatics – who complain its sacredness could be contaminated by "western decadent" books – and the intellectuals who believe the regime is intending to influence the publishing industry by holding the festival in a religious place.

Despite everything, schools and universities across the country bus students in for a cultural tour, creating a huge traffic jam in the packed streets of Tehran. Those coming from neighbouring provinces head to the capital early in the morning or even the night before to make the most of a day in the fair. It is more like a busy fruit market than a 10-day book fair.

"For me, it is the most exciting cultural event of the year," said Reza via online chat from Tehran. "In this country, we almost don't have any public carnivals to celebrate, we just have religious mourning ceremonies, they're all sad. The book fair is like a national celebration, like a carnival, it's absolutely fun."

Not everyone goes to the fair to buy books. In a country where male-female relationships outside marriage are not tolerated in public, boyfriends and girlfriends find the book fair the perfect meeting place. It's a good excuse to seek their parents' permission to get out and it's less likely they will get in trouble with the moral police. Scenes of boys and girls strolling aimlessly hand in hand, enjoying an ice-cream or sandwich, are common.

Some visit the fair several times. "Every year, I go twice. I usually buy so many books that I can't carry them home in one visit," said Ali, a physics teacher from Tehran. "The fair is so crowded and big that you can't visit all stalls in one visit anyway."

For publishing houses, Tehran's international book fair is simply a unique opportunity. Many sell more books in 10 days than they sell annually, albeit with tempting discounts. International publishing houses that participate in Tehran's fair are limited to those coming from friendly countries or those who publish books conforming to the strict rules of the country, such as scientific and medical books.

But not all Iranian companies are allowed to participate. This year, a number of prominent publishing houses have seen their licences revoked and have been banned from the book fair. This includes Cheshmeh, a prominent publishing house in Tehran specialising in literature and poetry books, which has released several titles by internationally known authors such as Katherine Mansfield, Mario Vargas Llosa, Toni Morrison, Paul Auster and Kazuo Ishiguro.

A recent crackdown on Iranian writers and publishers followed the country's supreme leader, Ayatollah Ali Khamenei's comments last year about "harmful" books and books "with a cultural appearance but with specific political hidden motives". In the face of the new measures taken against publishers, 160 acclaimed Iranian writers and translators signed a petition addressed to the country's ministry of cultural and Islamic guidance complaining about the new restrictions. Despite book censorship in Iran, writers and translators struggle to survive with some choosing to publish their work online for free. This saves them the task of seeking ministry permission to publish, which requires a book to be vetted by censors in a lengthy procedure.

Censorship

Iran's ministry of culture and Islamic guidance vets all books before publication. Three censors read each book to make sure it conforms to Islamic values. Censorship might apply to only a word, a sentence, a paragraph or sometimes a text as long as a dozen pages and the result would be given to the publisher after a long procedure that might last a year or two. Censors, who sometime use computer software to look up "unIslamic words", go as far as advising writers to substitute certain words with other "appropriate" phrases, should they wish their book to be approved. Publishing houses will be given negative points if they persist in sending too many books to the ministry which they deem to be unsuitable, encouraging self-censorship.

Speaking to the Guardian, Mehdi Navid, who has translated Richard Brautigan's In Watermelon Sugar into Persian, called some changes were ridiculous. When publishing a book by Charles Darwin on evolution, he said, the ministry asked the publisher to add an introduction to the book explaining that Darwin's views were unIslamic and untrue and the book was to be published to expose the wrongdoings and the decadence of the west.

Among words changed are cigarette (when used for a female character in a novel), laughing (again for female characters), swear, tattoo, cage and makeup (regardless of the character's gender).

"Kiss", "beloved", "wine" (in a non-fiction book about Charles Darwin, where it was mentioned that he got sick after drinking sour "wine" – the censor advised them to substitute the "wine" to a more appropriate word such as "juice"), "drunk", "pork", "dance", "rape", "dog" and "meditation" are among others frequently asked to be substituted. Male and female fictional characters are permitted to walk "hand-in-hand" in the story only if they are married couples. Censors will advise against any human touch between those fictional characters who are not married. Implicit gay touch can survive if the censor does not figure out that there is a homosexuality theme.

In reviewing a poetry book, one censor commented that it lacked appropriate rhythm. Another book, by celebrated writer Mahmoud Dolatabadi, was deemed too depressing.

Plato's Symposium, Louis-Ferdinand Céline's Journey to the End of the Night, Ulysses by James Joyce, Memories of My Melancholy Whores by Gabriel García Márquez, and some books by Kurt Vonnegut and Paulo Coelho are among books banned in Iran.